196 



terzaohi. SOFT-GROUND TUNNELING 



[Ch. 11 



under a pressure equal to the water pressure at the bottom of the 

 tunnel. In Fig. 16 the horizontal distances between ac and ab repre- 

 sent the pressure in the water which occupies the voids of the ground. 

 Since the air pressure on the heading is greater than the water pres- 

 sure, the compressed air not only stops the flow of water into the tun- 

 nel, but it also tends to drive the water out of the voids away from 

 the tunnel. The rate at which the water is displaced depends on the air 

 pressure and on the effective size Z) 10 of the soil particles. The value 

 of D 10 indicates that 10 percent of the total dry weight of the soil 



Ground surface 

 Water table s 



mmmv ^ Ammmmmw 



Water pressure 



Roof lining^ 



mmsmzm 



Air 



leakage 



Excess of air pressure 

 over water pressure 



(a) 

 Fig. 1. Diagram illustrating the principle of tunneling with compressed air. 



consists of particles with a grain size equal to or smaller than D 10 . 

 If Dio of the soil particles is smaller than about 0.01 millimeter, 

 the rate of displacement is likely to be zero, because the surface ten- 

 sion of the water may prevent the air from entering the voids. If Dio 

 is greater than about 0.01 millimeter, the rate of displacement rapidly 

 increases with increasing D 10 ; if D 10 is greater than about 0.2 milli- 

 meter, it becomes so high that a considerable quantity of air leaks 

 out of the tunnel. Under such conditions the miners are compelled to 

 plaster the joints in the temporary tunnel support and part of the ex- 

 posed surfaces of the ground with mud to reduce the loss of air. If 

 the heading strikes an exceptionally permeable layer, the air pressure 

 in the tunnel may suddenly drop to zero, whereupon the tunnel is 

 flooded. Such an accident is known as a blowout. In unconsolidated 

 deposits with an erratic structure, the danger of a blowout is consider- 

 ably greater than in regularly stratified ones, because they do not 

 contain continuous layers of relatively impermeable material which 

 interfere with the formation of air channels leading from the heading 

 toward the water table. 



